1. Field of the Invention
The present invention is directed to the field of internal combustion engine controllers and more particularly to that portion of the above noted field which is concerned with the provision of exhaust gas sensors for use in analyzing the chemical composition of exhaust gases. More particularly still, the present invention is concerned with an exhaust gas sensor support or probe, which will be capable of withstanding the exhaust gas environment for substantial periods of time without cracking or exhibiting other typs of fatigue failure, and with its method of manufacture.
2. Description of the Prior Art
Ceramic exhaust gas sensors of the electrically variable resistance type are known. For example, copending commonly assigned patent application Ser. No. 391,424 - "Air Fuel Ratio Sensor System" by H. L. Stadler et al. now U.S. Pat. No. 3,893,230 describes one such sensor fabricated from titania ceramic material and copending commonly assigned patent application Ser. No. 463,345 - "Air Fuel Ratio Sensor For Air Fuel Ratios In Excess Of Stoichiometry" now U.S. Pat. No. 3,933,028 describes such a sensor fabricated from cobalt monoxide ceramic material. Each of these materials demonstrates an electrical resistance change as a function of the partial pressure of oxygen in the gaseous environment of the ceramic material. The resistance changes may be measured by use of a pair of embedded electrodes. Each of these materials functions best at elevated temperature. However, the optimum operating temperature differs for the particular material. In order to operate the materials at their preferred operating temperature, the ceramic usually incorporates a heating means in the form of a platinum heater wire embedded within, or surrounding, the ceramic material. The embedding of the heater follows from the need to prevent any exothermic reactions from occurring at the surface of the platinum such as the catalyzation of carbon monoxide and the need to embed the resistance sensing leads.
In attempting to develop a sensor configuration which may be used in production as an exhaust gas sensor for automobile combustion engines, difficulties have been encountered in the laboratory in mounting the sensitive sensor ceramic material to a support or probe member in such a way that the sensor is reliably operable for periods of time sufficient to meet the demands of automotive production. The presently utilized method comprises bonding or cementing the sensor ceramic directly to a support ceramic with the electrical leads passing through the bonding material.
Sensor problems have arisen due to a variety of causes. In testing, sensor ceramics have become disunited from their support member. Also, vibrations induced by normal engine operation have operated to fracture the electrical wires at the surface of the ceramic sensor body. These problems are believed to stem, at least in part, from the fact that the resistive type ceramic exhaust gas sensor materials require a high degree of porosity in order to provide low response times. This high degree of porosity necessitates a mounting mechanism which places a minimum of the surface area of the sensor ceramic in masked contact with the support member.
A further problem has arisen from the fact that the prior exhaust gas sensor probes have utilized sensor ceramics which have placed a heater element physically within the sensor ceramic. Expansion due to, for example, differential rates of heating has caused nonuniform stresses on the sensor ceramic further complicating the mounting mechanism. It is therefore a specific object of the present invention to provide a probe for supporting an exhaust gas sensor ceramic of the resistive type in such a fashion that fracturing of the ceramic and/or of the electrical connections may be minimized. More particularly, it is an object of the present invention to provide such a structure in which the heating means may be physically distinct and separate from the sensor ceramic. More particularly still, it is an object of the present invention to provide a probe for supporting a wafer of exhaust gas sensor ceramic material which is capable of shielding the wafer of sensor material from direct exhaust gas flow impingement. It is a further object of the present invention to provide such a probe which will support a wafer of exhaust gas sensing material in gas flow contact with an exhaust gas stream while supporting a heating means in heat exchange relation with the exhaust gas sensor material without being in direct contact with the exhaust gas sensor ceramic. It is also an object of the present invention to provide a support member for a sensor ceramic which permits edge bonding between the ceramics without incorporating the electrical leads in the bond. It is a further object of the present invention to provide for edge bonding which does not require the use of cements or pastes.
As noted hereinabove, two types of resistive exhaust gas sensor ceramics are known to operate at temperatures which differ by several hundred degrees centigrade. It follows from the difference in temperature requirements that substantial differences in the sensor ceramic support mechanism can be expected. It is therefore a further object of the present invention to provide a ceramic support member for a resistive type exhaust gas sensor ceramic material which may be used with sensor ceramic materials which may require operation at widely different temperatures. It is also an object of the present invention to provide an auxiliary heating mechanism for assisting an exhaust gas sensor ceramic and its support probe to retain sufficient quantities of heat so that the exhaust gas sensor ceramic material may operate with the assistance of the auxiliary means at temperatures substantially in excess of that at which it would operate in the absence of the auxiliary means.